Molecules frequently have molecular resonance frequencies that are due to the electromagnetic attractions of atoms in the molecule. These frequencies are those of molecular vibrations, molecular rotational motions, the excitation of electrons to higher energy states, and occasionally finer structures such as hyperfine interactions and optical-magnetic properties. These properties are present without the introduction of any external contrast molecule. These frequencies are usually in the mid-infrared, corresponding to photons of 1.5-50 microns of wavelength. Unfortunately, they cannot be directly excited by electromagnetic radiation of the same frequency because when they are in tissue, the surrounding water absorbs almost all of these frequencies. The range of wavelengths that the tissue is relatively transparent to is 0.6-1.5 microns. Therefore multiphoton nonlinear processes need to be employed to probe these resonances. The photons to stimulate and record the processes are typically in a region where the tissue is not absorbing, so that they can reach the tissue feature and be measured from the feature.
Raman spectroscopy, first discovered in 1928, uses molecular resonance features of frequency Δω to split a photon of frequency ω into another photon of frequency ω−Δω and a resonance excitation of frequency Δω. The presence of photons at frequency ω−Δω identifies the concentration of the resonance feature. This process is in practice very weak and requires large amounts of power to produce any detectable amount of photons. This weakness is due to the fact that the probability of a Raman excitation process to occur is proportional to the number of photons at frequency ω−Δω already present, of which there are typically few or none. Since photons that would be emitted by Raman excitation at frequency ω−Δω are indistinguishable from the incoming radiation that stimulates them, this is not a viable technique for achieving molecular sensitivity.
Coherent Anti-Stokes Raman Scattering (CARS) is another nonlinear spectroscopy technique that unlike conventional Raman spectroscopy, allows all of the photons necessary to stimulate the process to be introduced into the tissue by the illuminating source. This enables the probability of a CARS interaction to be increased to a (theoretically arbitrarily) high level so that a sufficient number of photons can be produced as to enable detection within a reasonable time period. It is essentially two stimulated Raman scattering processes in parallel. Two photons, a “pump” of frequency ω1 and a “Stokes” of frequency ω2 illuminate the tissue. They must be separated in frequency by ω1−ω2=Δω, which is the frequency of the molecular resonance. When molecules of the target molecular species are present, the resonance will be excited, and the pump photon will be converted to the same frequency as the Stokes photon. This is the first stimulated Raman scattering process. Another photon may arrive at frequency ω3 that will stimulate the emission of the excitation from the resonance, so that the energy of the photon of frequency ω3 and the excitation are converted to a new photon of frequency ω4=ω3+Δω, called the “anti-Stokes” photon. The presence of this photon of frequency ω4 indicates that a CARS process has taken place and indeed a molecule with the resonance feature is present. Often the “pump” beam is used as the photons of frequency ω3, so that ω3=ω1 and ω4=2ω1−ω2. Since the photon of ω4 is not the same frequency as one of the illuminating photons, and is typically within the transparency range of the tissue, it is easily discriminated from the incoming radiation. FIG. 1 A shows an energy-level diagram for CARS, and FIG. 1 B shows an energy-level diagram for Coherent Stokes Raman Scattering (CSRS).
CARS microscopy uses the CARS process to look for the presence of a molecular species, but does not require any foreign substances to be introduced into the tissue. It scans the illumination point-by-point through the tissue and measures the number of generated anti-Stokes photons. When a three-dimensional mesh of points has been scanned, a complete three-dimensional picture of molecules of that resonance can be shown. Since CARS is a nonlinear process (and therefore is intensity sensitive), efficient conversion only occurs at the focus of the illumination, which can be made very tight (typically a half micron in both the axial and lateral directions). Therefore the resolution can be made many orders of magnitude better than MRI, which is the probably the largest competition for clinical use for similar purposes. Unfortunately, the penetration is usually rather low (limited to about 500 microns). A further shortcoming is that CARS microscopy measures the total number of anti-Stokes photons, or power, from the sample. However, the optical field contains temporal structure in the phase that is averaged out by power detection because photodetector response time is orders of magnitude slower than the oscillations of the optical field. The time scale on which the optical pulse varies (which is typically picoseconds or femtosecond time scales) is far too fast for photon detection equipment or electronics to detect (the fastest of which may detect 25 ps time scales).
Optical coherence tomography (OCT) is an emerging high-resolution medical and biological imaging technology. OCT is analogous to ultrasound B-mode imaging except reflections of low-coherence light are detected rather than sound. OCT detects changes in the backscattered amplitude and phase of light.
Nonlinear interferometric vibrational imaging (NIVI) is a method used to measure the three-dimensional distribution of molecular species in various samples (biological or otherwise) [1]. Its basic operation is to stimulate the excitation of molecular bonds with particular resonance frequencies, and then use these excitations to produce radiation distinct from the excitation that can be measured. Unlike previous methods that use CARS in microscopy to probe for the presence of molecular species, NIVI utilizes a heterodyne approach where a reference signal is separately generated and interferomerically compared to the signal received from the sample, allowing the signal to be formed into an image in the same way as OCT. In this way, additional information can be inferred from the emitted radiation such as the distance to the sample and phase information that yields additional structure of the molecular bonds. It also has other advantages in sensitivity and the ability to screen out background radiation that is not produced by the sample. It also can allow more flexibility in the choice of laser illumination source, because the coherent detection process does not rely on photon frequency alone to discriminate emitted radiation.
There are compelling reasons to use broadband sources to excite CARS. An ultrafast pulse can be shaped into a longer picosecond pulse that can excite CARS more efficiently. In addition, it can be used to excite many resonances simultaneously [2]. Unfortunately, unlike narrowband pulses, the anti-Stokes radiation produced will not be narrowband. If many resonances are excited simultaneously then the anti-Stokes radiation they produce have overlapping spectra. Because noninterferometric detection can only measure the spectrum of the anti-Stokes radiation, the contributions of each resonance to the anti-Stokes radiation will be inseparable. Interferometric detection allows the demodulation of the anti-Stokes field so the Raman spectrum can be inferred when exciting multiple simultaneous resonances.
In addition, broadband sources should allow for more Raman-frequency agile imaging instruments. When utilizing narrowband pump and Stokes pulses, the frequency difference between them must be tuned to the Raman frequency of interest. Often retuning lasers or amplifiers is difficult to make reliable and automatic. Pulse shaping, however, is achieved by movable gratings, prisms, or mirrors to adjust dispersion and delay, or by acousto-optic or liquid-crystal Fourier plane pulse shapers without moving parts [3]. Because pulse shapers do not typically involve feedback, oscillation, or overly sensitive alignment, pulse shapes can be changed much more easily. In addition, a computer can control these mechanisms automatically, so that changing the pulse shape should be much easier than retuning a laser source.
Two important features of the Raman spectrum in practice that need to be mutually distinguished are the resonant and nonresonant components. The resonant components are specific to features of a molecule, which include vibrational-frequencies, rotational frequencies, and electronic resonances. Nonresonant features are not specific to a particular molecule, and are weakly dependent on frequency.
Distinguishing CARS resonant from nonresonant four-wave-mixing has become problematic with the advent of ultrafast laser sources. These sources produce pulses on the order of 5-200 fs, much shorter than the lifetime of the resonance, which is 1/Γn. If transform-limited ultrafast pulses are used, only a small polarization P(3)(Ω) can be produced in the molecule, while the nonresonant components are enhanced. Since the lifetime of the resonance is typically 1-100 ps, transform-limited ultrafast pulses excite resonant transitions inefficiently and nonresonant transitions efficiently. Before the advent of near-infrared solid-state femtosecond lasers, especially the Ti3+:Al2O3 laser, most laser sources produced picosecond-length pulses that favored the generation of CARS. Many current CARS instruments utilize narrowband pump and Stokes pulses of picosecond length for this reason.